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Asmerom, Y. and Jacobsen, S.B. (1993). The Pb Isotopic Evolution of the Earth - Inferences from River Water Suspended Loads. Earth and Planetary Science Letters 115(1-4): 245-256. |
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Pb and U concentrations and Pb isotopic variations in the suspended loads of major rivers draining regions with a range of ages of crustal formation are reported. The Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 ratios are positively correlated with Sr-87/Sr-86 and negatively correlated with epsilon(Nd) values in the samples. A coherent negative correlation of the Pb isotope ratios with epsilon(Nd) values allows us to estimate the average Pb isotope composition of the upper continental crust at Pb-206/Pb-204 = 19.32 +/- 0.28, Pb-207/Pb-204 = 15.76 +/- 0.09 and Pb-208/Pb-204 = 39.33 +/- 0.39. The Pb-206/Pb-204 ratios are positively correlated with their Nd model ages, reflecting the higher upper continental crust mu values relative to mantle mu values. The upper continental crust, as well as the MORB and OIB sources, plot to the right of the geochron. Assuming that the lower crust is the missing low-mu reservoir, we have calculated a range of possible lower crust Pb isotope compositions similar to many granulite terrains. Overall, the upper continental crust has somewhat higher Pb-207/Pb-204 than MORB or OIB, while Pb-206/Pb-204 overlaps with the MORB values and is less radiogenic than some OIB values. Typical measured mu values for the suspended loads of large rivers are approximately 4, while the mu values of their crustal source rocks (based on the Pb isotope composition) are approximately 12, allowing for up to 65% of U from continental weathering being carried in the dissolved load to the oceans. The near-overlap of the Pb-206/Pb-204 values of MORB and continental crust is consistent with the suggestion that U is recycled into the mantle through altered oceanic crust. The exception to these trends are the samples with the oldest Nd model ages (approximately 3.5 Ga). The Pb isotope ratios show drastic deviation from the Pb and Sr correlation trend suggesting an inherent low mu value in the mantle source of the early Archean continental crust. |
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Keywords
uranium-thorium-lead, mantle evolution, terrestrial lead, west-greenland, element abundances, oceanic-crust, world rivers, island arcs, nd, sr |
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Publisher
Elsevier Science
P.O. Box 211
1000 AE Amsterdam
The Netherlands
(+31) 20 485 3757
(+31) 20 485 3432
nlinfo-f@elsevier.com |
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